BNC 2110 with PXIe 6366 error in readings

[matt87]

Hey everyone.

I have a 1073-chassis with a 6366 PXIe DAQ, and am using a BNC-2110 connector block. I have downloaded all the new drivers and my DAQ is able to support the connector block.

Howver, I noticed when I was measuring data that when I had 2 channels measuring simultaneously my results for both channels would shift severely. When I removed either channel the values looked like what I would expect again. ANy suggestions on how I may solve this issue?

Regards,

Mathew Clark

[Michelle_N]

Hi Mathew,

Can you send us screenshots of the signals - what they look like when read individually and what they look like when read simultaneously? Also, if you can post the code you are using, that would be great.

[matt87]

I can do that. But I want to point out that I was looking for more of a hardware solution to this issue, as this occurs when I physically remove the probes. The code doesn't do anything but sample the data and store it in binary format.I will post the signal and the code shortly, if there is demand for it.

[Seth_B.]

Seeing the signal helps us understand what the hardware is seeing.  Seeing the code shows us the software configuration of the hardware, which is also relevant as it controls how the device is being configured internally.  Additionally, are your signal sources floating or grounded?  And are the switches on the BNC-2110 set to "FS" or "GS"

[matt87]

Hey Seth,

I have been searching around a bit more, and came up with something interesting. As to your question, I have the BNC set to "FS" as I presumed the "GS" would create a ground-loop and add too much noise and error to my signal.

Having that said, I realized that the code runs in "differential" mode in my LabVIEW vi (thanks for pointing out i should scrutinize the code closer). Now, I read that "pseudodifferential" seems more viable for a Multifunction DAQ, but when I implement it my code doesn't function anymore and says that I can only select differential mode.

I have attached the code.

In terms of signal, I have no output available other than the numbers.

I read 79 mA with one channel connected, but 75 mA with 2 channels connected on the other channel

I read 10 mA with one channel connected, but 8.8 mA with 2 channels connected.

Regards,

Mathew

[John_P1]

The FS / GS switch does the following according to the manual:

So, in FS mode you're actually connecting AI GND to the (-) input of your signal source through a 5 kOhm bias resistor.  If your signal source is floating this is necessary to prevent excess charge from building up on the ADC--there wouldn't be a ground loop in this case since the signal source itself is floating and will be tied to just your AI GND.  If your signal sources are grounded, you should use the GS setting to avoid the ground loop.  More information about connecting signals can be found here.

I'm not sure where you read about pseudodifferential, but this just means that the bandwidth of ai(-) is much less than that of ai(+).  In a truly differential input (like the 6366), the bandwidth is the same.  Some other DAQ devices (mostly S Series) have the pseudodifferential inputs.  It's true that the 6366 only supports differential mode.

What sensors are you connecting to the inputs?

Best Regards,

[matt87]

Hi John,

I have my probes connected along 0.1 ohm sense-resistors in order to measured current along a board. I understand what you're saying, when I set everything to "GS" my values go way out of range, that's why I have kept it at "FS". In terms of the pseudo-differential, that was something I came across when I was looking into simultaneous sampling (which I am trying to do), but I wasn't sure how much relevance it had.

Regards,

Mathew

[John_P1]

Hi Matthew,

Could you provide a circuit diagram?  It sounds like you are probing a circuit at different locations to determine the current, is this correct?  What's the common-mode voltage (relative to AI GND) of each of the probes?  What do the signals look like when  you connect the two channels at the same time vs. independently?

AI+ and AI- can't exceed 11V from AI_GND.  I'm assuming that this happens when you have your switches on the 2110 set to GS.  Setting the switches to FS connects the AI- references to AI GND through a 5 kOhm resistor.  By extension, it also connects the AI- references together through 10 kOhm of total resistance.  Evidently this is enough to create an offset on your channels--to explain further I'd have to see what the circuit you are connecting actually looks like.

Based on the situation that you've described so far, my recommendation would be to use an external buffer to reference your channels to the same voltage level (AI GND) without having to worry about affecting the resistance of your circuit or the common mode differences between the two channels.

Best Regards,